Early Sport Specialization Linked to Throwing Arm Function and Upper Extremity Injury History in College Baseball Players

Abstract

Background:

Evidence suggests that shoulder and elbow injuries account for 31% to 37% of all National Collegiate Athletic Association (NCAA) baseball injuries, and up to 69% of NCAA baseball injuries are the result of noncontact and overuse mechanisms. Early sport specialization may contribute to the high rates of upper extremity injuries in college baseball players.

Hypothesis:

Higher specialization by age 13 years would be associated with worse subjective throwing arm function and a greater history of shoulder and elbow injury.

Study Design:

Cohort study.

Level of Evidence:

Level 2.

Methods:

Survey data were collected from college baseball players (N = 129) during midseason of the spring 2019 baseball season. Participants were stratified in low, moderate, and high specialization groups based on a 3-criteria sports specialization questionnaire. Participants’ throwing arm function was measured using the Functional Arm Scale for Throwers and the Kerlan-Jobe Orthopaedic Clinic shoulder and elbow questionnaires. Participants’ history of a shoulder or elbow injury that resulted in missing ≥2 weeks of baseball activity at any point in their baseball career was also collected.

Results:

The high specialization group reported worse subjective throwing arm function on the Functional Arm Scale for Throwers questionnaire than the low (P = 0.03) and moderate (P = 0.01) specialization groups. The high specialization group was over 5 times more likely to report a history of shoulder injury than the moderate (odds ratio [OR] = 5.42; 95% CI [1.71, 17.2]; P = 0.004) and low (OR = 5.20; 95% CI [1.87, 14.5]; P = 0.002) specialization groups, and over 3 times more likely to report a history of elbow injury than the moderate specialization group (OR = 3.77; 95% CI [1.05, 13.6]; P = 0.04).

Conclusion:

Clinical Relevance:

Early specialization in baseball may be detrimental to long-term upper extremity health in college baseball players.

Keywords

shoulder injury elbow injury overhead athlete youth baseball

The upper extremity is the most common injured body region in collegiate baseball players, with recent epidemiologic reports suggesting that injuries to the shoulder and elbow account for approximately 37% of all practice injuries and 31% of all competition injuries, among National Collegiate Athletic Association (NCAA) baseball players.²⁰ The most common mechanisms of injury in NCAA baseball players are noncontact and overuse injuries, which account for approximately 69% and 49% of injuries during practices and competition, respectively.²⁰ Interestingly, we see similar injury patterns among high school and youth baseball players as well. Reports suggest that up to 31% of high school baseball injuries are to the shoulder and elbow²⁰ and that noncontact and overuse mechanisms account for 71% to 74% of high school baseball shoulder and elbow injuries.¹⁶ Even in youth baseball players aged 7 to 12 years, evidence suggests that 16% experienced an episode of shoulder pain and 29% experienced elbow pain that caused them to miss baseball activity during a season.¹⁰ It is possible that the high rates of noncontact and overuse shoulder and elbow injuries observed in collegiate and young baseball players may be the consequence of years of repetitive training and tissue stress starting as early as youth baseball.

Over time from youth through collegiate baseball, the repetitive and unique mechanics of overhead throwing may increase the risk of upper extremity injuries and dysfunction in collegiate baseball players. During the throwing motion, specifically the late cocking and acceleration phases, the upper extremity tissues of the shoulder and elbow are exposed to elevated torques. At the shoulder, limited bony stability combined with high-velocity movements during the throwing motion places excessive stress on the rotator cuff, surrounding musculature, and glenoid labrum to maintain glenohumeral joint stability.¹³ At the elbow, there is a significant increase in extension and valgus torque during the acceleration and follow-through phases that puts a great amount of stress on the medial and posterior parts of the elbow.^12,21 Over time, these repetitive stresses could lead to deterioration and maladaptation in musculoskeletal tissues of baseball players.¹¹ In youth baseball players, these stresses may have even greater detrimental effects on upper extremity tissues due to increased bone plasticity, ligamentous laxity, open epiphyseal growth plates, and underdeveloped musculature.¹³

Sport specialization, defined as year-round intensive training in a single sport while excluding other sports,⁷ is a popular trend among youth baseball players.² The high-volume training, repetitive tissue stresses, and limited rest associated with sports specialization could play a role in the high rates of noncontact and overuse upper extremity injuries we have observed in college baseball players. A recent meta-analysis suggests that high and moderate sport specialization during adolescence is associated with a greater risk for overuse injury across diverse sports.³ In a recent study of youth baseball players younger than 13 years, 83% of players met the study’s criteria to be considered specialized in baseball, and those who were specialized reported a higher frequency of throwing-related arm injuries compared with players who were not specialized.² Similarly, in another recent study of high school baseball players, 52% of players met the study’s criteria of being highly specialized, and those who were highly specialized reported greater rates of upper extremity overuse injuries in the previous year and poorer subjective throwing arm function.¹⁴

Evidence suggests that adolescent baseball specialization may be associated with short-term negative effects on upper extremity health during players’ adolescent baseball careers^2,14; however, the long-term effects of early baseball specialization on upper extremity health are not well-understood. It is possible that early baseball specialization may contribute to the high rates of overuse and noncontact shoulder and elbow injuries that have been reported in collegiate baseball players. Therefore, the purposes of our study were to compare current subjective throwing arm function and shoulder and elbow injury history in college baseball players who reported high, moderate, and low specialization by the age of 13 years. The authors hypothesized that players who were highly specialized in baseball by age 13 years would report poorer current throwing arm function and a greater history of upper extremity injury than players who were low specialization.

Methods

Study Design

Our study utilized a retrospective cohort design. Survey data were collected from all participants at their respective practice facilities during midseason of the spring 2019 baseball season. Our primary independent variable was participants’ baseball specialization level (high, moderate, or low) by the age of 13 years. Previous evidence observed that specialization prior to age 13 years was associated with high rates of upper extremity injury,² and age 13 years represents a stage of early adolescence prior to the start of high school baseball activities. Our primary dependent variables included participants’ current subjective throwing arm function, measured using the Functional Arm Scale for Throwers (FAST)¹⁷ and the Kerlan-Jobe Orthopaedic Clinic (KJOC)⁸ shoulder and elbow scores, and participants’ history of shoulder injury and elbow injury. Our college’s institutional review board approved this study, and all participants provided informed consent.

Participants

After recruiting 136 college baseball players between the ages of 18 and 22 years, our sample included 129 players from NCAA Division II (n = 30), NCAA Division III (n = 49), National Association of Intercollegiate Athletics (n = 34), and National Club Baseball Association Division II (n = 16) baseball teams at southeastern Michigan colleges in the United States (Table 1). Seven participants were excluded because they had not participated in baseball-related activities in the 1 week prior to data collection.

Table 1.

Participant demographics by specialization levels^a

	High (n = 22)	Moderate (n = 38)	Low (n = 69)	Total (N = 129)
Age, y, mean ± SD	19.9 ± 1.2	19.7 ± 1.4	19.9 ± 1.1	19.8 ± 1.2
Mass, kg, mean ± SD	93.3 ± 9.4	92.3 ± 12.2	91.5 ± 10.9	92.1 ± 11.0
Height, m, mean ± SD	1.8 ± 0.06	1.8 ± 0.07	1.8 ± 0.07	1.8 ± 0.07
Baseball years, y, mean ± SD	13.8 ± 3.3	12.7 ± 4.0	13.3 ± 3.0	13.2 ± 3.3
Position, n (%)
Pitcher	9 (40.9)	12 (31.6)	28 (40.6)	49 (38)
Nonpitcher	13 (59.1)	26 (68.4)	41 (59.4)	80 (62)

No significant differences between high, moderate, and low groups (all Ps >0.05).

Procedures

The authors collected data during a formal team meeting at each team’s respective facilities. Participants were provided a series of questionnaires: a demographics questionnaire, 3-criteria specialization questionnaire, shoulder and elbow injury history questionnaire, and the FAST and KJOC questionnaires. The demographics questionnaire included questions about the participant’s age, weight, height, years of organized baseball participation, and primary baseball position.

3-Criteria Specialization Questionnaire

The 3-criteria sport specialization questionnaire was developed to categorize athletes as high, moderate, or low sport specialization.⁶ In the current study, participants provided the age (years) at which they met each of the criteria. Participants were asked to provide an age or answer “never” to each of the following questions: (1) At what age did you train for more than 8 months out of the year in baseball? (2) At what age did you consider baseball more important than other sports? (3) At what age did you quit other sports to focus on baseball? Based on the ages provided, the authors were able to determine at what ages participants met each of the 3 sport specialization criteria and then to categorize participants as high (3 criteria), moderate (2 criteria), or low (0-1 criterion) specialization for each age. This was accomplished with the assumption that each criterion was met by the age provided and for each subsequent year thereafter, but not the years before. For example, if participants indicated they starting training for more than 8 months out of the year in baseball at age 13 years, they did not meet that criterion for ages 12 years and younger but did meet the criteria for ages 13 years and older. This approach allowed the authors not only to perform our analyses based on baseball specialization groups by age 13 years but also to descriptively examine specialization across participants’ adolescent baseball careers (Figure 1).

Injury History and Throwing Arm Function

Subjective throwing arm function was assessed using the FAST¹⁷ and KJOC⁵ questionnaires. The FAST consists 22 questions based on the participant’s throwing arm condition during the last week of throwing and daily activities. For each question, answers are provided on a 5-option Likert-type scale and scored from 1 to 5 points. The FAST total score is summed and normalized on a 0% to 100% scale, with a lower score indicating lower function. The KJOC includes 10 questions regarding the participant’s throwing arm function during game and practice conditions. Participants answer each question by marking their response on a 10-cm visual analog scale, and each visual analog scale is oriented so a lower score represents lower function. The KJOC total score is calculated on a 0 to 100 scale by summing the scores for the 10 questions, with a lower score indicating lower function.

Injury history was determined by asking whether participants ever had a shoulder injury or elbow injury that had caused them to miss at least 2 weeks of baseball activity (yes or no). These questions did not include a specific time frame for injury, so injuries could have occurred during the participant’s youth, high school, or college baseball career.

Data Analysis

Descriptive data was presented as means ± SDs, medians [interquartile ranges], and frequencies (proportions). Participant age, mass, height, years of baseball played, and baseball position between the high, moderate, and low specialization groups using 1-way analyses of variance and a chi-square test (position) was compared. FAST and KJOC data were nonnormally distributed, so scores were compared between the high, moderate, and low specialization groups using separate Kruskal-Wallis tests with post hoc Mann-Whitney U tests and robust effect sizes (d_r)¹ with 95% CIs. Robust effect sizes were interpreted as small (0.20-0.49), moderate (0.50-0.79), and large (>0.80).^9,18 Shoulder and elbow injuries were compared between the 3 specialization groups using initial (3 × 2) and post hoc (2 × 2) chi-square tests. The authors used binary logistic regression to calculate odds ratios (ORs) with 95% CIs to compare the likelihood of reporting a shoulder injury or elbow injury in the high specialization group versus low and moderate specialization groups. All analyses were considered statistically significant with an α-level of P < 0.05 and performed using SPSS statistics 27 software (IBM Corporation).

Results

By age 13 years, 17.0% of participants demonstrated a high level of specialization, 29.5% demonstrated a moderate level of specialization, and 53.5% demonstrated a low level of specialization (Figure 1). By age 13 years, 59.7% (77/129) considered baseball more important than other sports, 50.4% (65/129) trained for more than 8 months of the year in baseball, and 25.6% (33/129) had quit other sports to focus on baseball. No significant differences were observed in age (P = 0.68), mass (P = 0.79), height (P = 0.83), years of baseball participation (P = 0.44), or baseball position (P = 0.63) between the high, moderate, and low specialization groups (Table 1).

The proportion of participants classified as high, moderate, and low specialization at ages 10 to 18 years of their adolescent baseball careers is presented in Figure 1. Over the course of participants’ adolescent baseball career, the average ages of high and moderate specialization were 14.8 ± 2.3 and 13.3 ± 3.1, respectively, with 22.5% of the sample never meeting the criteria for high specialization and only 3.1% never meeting the criteria for high or moderate specialization by age 18 years. The proportion of participants who met each sport specialization criteria at ages 10 to 18 years is presented in Figure 2. Over the course of their adolescent baseball careers, participants considered baseball more important than other sports at an average age of 11.9 ± 3.8 years, played baseball for greater than 8 months of the year at an average age of 13.3 ± 2.9 years, and quit other sports to focus on baseball at an average age of 13.7 ± 3.5 years.

Significant differences were observed in FAST scores (P = 0.01) between the 3 specialization groups but no significant difference in KJOC scores (P = 0.12) (Table 2). Specifically, the high specialization group reported lower FAST scores, suggesting poorer throwing arm function, than the moderate (P = 0.01; d_r = 0.62; 95% CI [0.08, 1.15]) and low (P = 0.03; d_r = 0.39; 95% CI [−0.09, 0.88]) specialization groups, but there were no differences in FAST scores between the moderate and low specialization groups (P = 0.12; d_r = 0.22; 95% CI [−0.18, 0.61]).

Table 2.

Shoulder injuries, elbow injuries, and subjective throwing arm function in low, moderate, and high specialization groups

	High (n = 22)	Moderate (n = 38)	Low (n = 69)
FAST, median [IQR]	81.3 [13.4]^a	90.4 [18.5]	87.5 [15.9]
KJOC, median [IQR]	72.5 [34.3]	84.7 [26.2]	87.2 [19.2]
Shoulder injury, n (%)	13 (59.1)^b	8 (21.1)	15 (21.7)
Elbow injury, n (%)	8 (36.4)	5 (13.2)	22 (31.9)

FAST, Functional Arm Scale for Throwers; IQR, interquartile range; KJOC, Kerlan-Jobe Orthopaedic Clinic.

Significantly lower FAST score than moderate (P = 0.01) and low (P = 0.03) specialization groups.

Significantly greater shoulder injuries than the moderate (P = 0.003) and low (P = 0.001) specialization groups.

Significant differences were observed in shoulder injury history between the 3 specialization groups (P = 0.002) (Table 2). The high specialization group reported significantly greater rates of shoulder injury compared with the moderate (P = 0.003) and low (P = 0.001) specialization groups. There were no differences in shoulder injury history between the moderate and low specialization groups (P = 0.93). The high specialization group was over 5 times more likely to report a history of shoulder injury than the moderate (OR = 5.42; 95% CI [1.71, 17.2]; P = 0.004) and low (OR = 5.20; 95% CI [1.87, 14.5]; P = 0.002) specialization groups.

No significant differences were observed in elbow injury history between the 3 specialization groups (P = 0.07) (Table 2). However, the high specialization group was over 3 times more likely to have a history of elbow injury than the moderate specialization group (OR = 3.77; 95% CI [1.05, 13.6); P = 0.04) but had no greater odds of an elbow injury than the low specialization group (OR = 1.22; 95% CI [0.45, 3.34]; P = 0.70).

Discussion

In alignment with the hypotheses, college baseball players who were highly specialized in baseball by the age of 13 years reported worse subjective throwing arm function on the FAST questionnaire and were over 5 times more likely to have a history of shoulder injury than college baseball players who reported moderate or low specialization by 13 years. The difference in FAST score between the high and moderate specialization groups was supported by a moderate effect size, while the difference between the high and low specialization groups was supported by a smaller effect size. In contrast with our hypotheses, the authors observed no significant differences in rates of elbow injury or KJOC scores between the 3 specialization groups; however, ORs suggested the high specialization was over 3 times more likely to report a history of elbow injury than the moderate specialization group. Collectively, the findings suggest that high specialization by age 13 years was associated with worse subjective throwing arm function and greater odds of upper extremity injury than moderate and low specialization; however, this effect seemed to be greatest when comparing high specialization with moderate specialization, not low specialization, in our sample.

Our findings are in alignment with previous studies examining the relationships between specialization and upper extremity injury in youth, high school, and professional baseball players.^2,14,22 Two studies in youth and high school baseball players examined the short-term relationship between specialization and concurrent upper extremity injury and function. In a prospective study of youth baseball players younger than 13 years (N = 159), currently specialized players reported higher rates of upper extremity injuries (impairment that required the player to miss >1 practice or change his position) over a 6-month period than those who were nonspecialized.² In a retrospective study of high school baseball players (N = 551), players who were highly specialized during high school were about 3 times more likely to report an upper extremity overuse injury in the past year and also reported worse current throwing arm function than those that were low specialization.¹⁴ In a retrospective study of professional baseball players (N = 102), players that reported specializing in baseball prior to high school also reported a higher frequency of injuries that resulted in >1 year time lost during their professional baseball careers than those that were not specialized prior to high school.²² Interestingly, each of these studies,^2,14,22 as well as this study, examined the relationship between baseball specialization and upper extremity injury using different populations and alternative methods for measuring specialization, upper extremity injury, and throwing arm function, but have all come to similar conclusions. Collectively, this body of evidence suggests that early adolescent baseball specialization may be associated with concurrent and long-term detrimental effects on upper extremity health.

In our sample, the average age of high specialization was almost 15 years, suggesting that our participants who were highly specialized by age 13 years were doing so almost 2 years earlier in their development than their average peer. Given our findings that high specialization prior to the age of 13 years may be detrimental to long-term upper extremity health, it was encouraging that only 17% of our sample met the criteria of high specialization and only 47% met the criteria for either moderate or high specialization at age 13 years; however, previous studies have reported specialization rates of 83% in youth baseball players aged 9 to 12 years.² Our descriptive examination of the rates of high, moderate, and low specialization rates in participants from ages 10 to 18 years suggests a potential exponential rise in high specialization rates starting at about the age of 12 years and continuing through to age 18 years. Based on the average ages that each specialization criterion was met in the sample, the transition to higher levels of baseball specialization may first begin with considering baseball more important than other sports, followed by training for more than 8 months of the year in baseball, and finally quitting other sports to focus on baseball. These stages could be valuable when looking for signs that a youth baseball player may be starting to specialize.

The findings of this study could be useful for clinicians when educating youth baseball patients and parents/guardians about the potential negative consequences of early specialization, particularly for those interested in a college athletic career. Theoretically, specialized patients could be identified by pediatricians and athletic trainers using the 3-criteria sport specialization questions during annual wellness and preparticipation examinations. Similar to identifying any other exposure in an adolescent patient’s history, the clinician could then educate patients and parents/guardians on the potential consequences of their behavior and discuss alternative options. However, there is currently no evidence examining the effectiveness of patient or parent/guardian education on deterring specialization practices.

Baseball is one of the few sports that has instituted rules in an effort to minimize upper extremity injuries; however, rules alone may not be enough to protect the long-term health of youth athletes. The Pitch Smart Program was established in 2006 by the combined effort of Major League Baseball and USA Baseball to combat the epidemic of youth baseball injuries; however, there are few studies that have evaluated the effectiveness of these guidelines in the prevention of throwing injuries in adolescent pitchers.¹⁵ Interestingly, the average player in this sample was born in the year 2000 and therefore would have grown up playing youth baseball with these pitch-count rules. Many US state high school baseball leagues have instituted pitch-count limits and/or rest mandates for pitchers in alignment with the Pitch Smart recommendations; however, these rules vary by state and are not instituted across all US states.⁴ Limitations of pitch-count rules include not accounting for non–pitching related throwing and not accounting for throwing and pitching outside of league competition (eg, private lessons, club teams). A recent study observed that during a season, 11- to 12-year-old baseball players performed about 10 times more nonpitching throws and about 2 times more high-effort nonpitching throws compared with pitching throws,¹⁹ suggesting that pitch counts alone underestimate the total stresses that a player’s throwing arm experiences during youth baseball. While the authors observed no significant difference in the number of current pitchers between our high, moderate, and low specialization groups, the authors did not ask about participants’ pitching history during adolescence, which limits our ability to draw conclusions about the relationship between early baseball specialization and upper extremity health relative to pitching.

This study has significant limitations. Since we used a retrospective design, our data relied heavily on participant recall for both their specialization criteria ages and injury history. Additionally, it is difficult to establish a true cause-effect relationship between early baseball specialization and long-term upper extremity health using a retrospective design. This sample was also relatively small and only included baseball players from small colleges in southeastern Michigan, which did not include participants representing NCAA Division I baseball players, thus limiting the applicability of these findings to the broader collegiate baseball population. This study also had a smaller representation in of participants in the high specialization group (n = 22), which may have biased some of our statistical analyses.

Conclusion

College baseball players that were highly specialized by age 13 years reported worse subjective throwing arm function and were more likely to have a history of upper extremity injury than players that were moderate or low specialization. Early specialization in baseball may be detrimental to long-term upper extremity health in college baseball players.

Footnotes

The authors report no potential conflicts of interest in the development and publication of this article.

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